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Grass condition is more of a factor in grazing behviour than lactation status according to research unveiled by Kansas State University.

Introduction

Diet selection is a dynamic process because of seasonal changes in animal and plant
characteristics. Nutrient requirements of grazing animals are a function of physiological
state; moreover, plant characteristics may be altered with prescribed spring burning of
native rangelands. Prescribed spring burning is used to improve the average quality of
pasture forage by removing old growth and making new plant growth more accessible to
grazing cattle.

Microhistological analysis of fecal material has been a widely used method for quantifying
the botanical composition of a grazing animal’s diet since it was first described by
Baumgartner and Martin in 1939. Little research has been conducted on how diet selection
preferences of lactating beef cows with suckling calves and non-lactating beef cows
are influenced by prescribed burning. We hypothesized that during the summer grazing
season, lactating cows with calves and non-lactating cows would display distinctive
preferences for certain species. Furthermore, we anticipated that these diet selection
preferences might be influenced by prescribed burning. To that end, our objective was
to characterize differences in diet selection between lactating beef cows suckling calves
and non-pregnant, non-lactating beef cows grazing either burned or unburned native
tallgrass prairie during summer.

Experimental Procedures

The study was conducted on 8 native tallgrass pastures (approximately 240 acres
each) located at the Kansas State University Commercial Cow-Calf Unit. Four of the
pastures were burned in mid-April and 4 had no recent burning history. Predominant
pasture forage species at this location were big bluestem (Andropogon geradii) and little
bluestem (Schizachyrium scoparium), which were grouped together for the purposes of
microhistological analysis; sideoats grama (Bouteloua curtipendula); blue grama (Bouteloua
gracilis); switchgrass (Panicum virgatum); indiangrass (Sorghastrum nutans); leadplant
(Amorpha canescens); heath aster (Symphyotrichum ericoides); dotted gayfeather
(Liatris punctata); and purple prairie clover (Dalea purpurea). Grazing commenced
May 15.

Results and Discussion

Previous results suggested that lesser maintenance requirements could result in less
selective foraging behaviors by non-lactating compared with lactating ruminants. Previous
research also indicated lactating cows grazed more selectively than non-lactating,
non-pregnant cows; however, we found no treatment differences (P ? 0.11) in the
botanical diet composition between lactating and non-lactating cows (Table 1). Similar
findings were reported that found no differences in diet composition between lactating
ewes and non-lactating ewes.

As the grazing season progressed, selection of switchgrass increased (burn × period
effect, P = 0.09) sharply in both burned and unburned pastures, whereas selection of
sideoats grama generally decreased (burn × period effect, P < 0.01; Table 3). Selection of leadplant doubled (burn × period effect, P = 0.04) on burned pastures monthby-
month, but selection was inconsistent in unburned pastures. Selection of dotted
gayfeather ranged from 12.3 to 20.4% of the diet in June, July, and August and diminished
to 8.5 to 8.9% in September (burn × period effect, P = 0.05).

Cows selected more (P < 0.01) switchgrass, blue grama, leadplant, and heath aster over
time, whereas they selected less (P < 0.01) indiangrass over time (Table 4). Palatability
is a major factor driving selection preferences by grazing herbivores and is reduced
as plants approach reproductive maturity and dormancy. Under unrestricted grazing
conditions, herbivore preference for specific forage plants is known to change over time.
The cows used in our study may have modified their diets over time to select greater
proportions of plants that were slower to reach maturity. Alternatively, decreased
consumption over time may have been related to diminishing availability or regrowth of
certain forage plants.

Consumption of all grasses and all forbs changed slightly (P < 0.01, Table 4) from
month to month during the grazing season; however, the relative proportions of grasses
and forbs remained consistently within the range of 71 to 75% grasses and 25 to 29%
forbs.

Effect of Collection Period on Botanical Composition of Diets (%) Selected by Lactating Cows with Calves or
Non-lactating, Non-pregnant Cows Grazing the Kansas Flint Hills during Summer

Implications

The botanical composition of diets grazed by beef cows during summer in the Kansas
Flint Hills was influenced by prescribed spring burning but was not influenced by lactation
status. We interpreted these data to suggest that forage selection preferences of
beef cows can be altered with spring burning of native tallgrass pastures.